1. Field of the Invention
The present invention relates to an active matrix type liquid crystal display apparatus having a thin film transistor (TFT) and to a TFT panel.
2. Description of the Background Art
Referring to FIG. 28, a TFT panel includes a display area 102 and a peripheral area therearound, on a transparent substrate 101. The peripheral area consists of an X-direction driving circuit portion 111, an Y-direction driving circuit portion 112, and an external connection terminal portion 117. In each driving circuit portion, driving transistors (not shown) are arranged, each of which controls and drives a pixel transistor (not shown) arranged at an intersection of a signal line and a scanning line (both not shown) in the display area. The main feature of a liquid crystal display apparatus is that it is thin and compact. It is a main attraction for a consumer purchasing the apparatus. If the driving transistors arranged in, respective driving circuit portions can be accommodated in a compact manner, a frame of the display screen can be made narrower, and hence commercial value will be increased. In this respect, the arrangement of the driving transistors in the X-direction driving circuit portion 111 in areas 115 and 116 of FIG. 28 is important.
In a TFT panel in which the display area and the driving circuit areas are integrated, polycrystalline silicon (hereinafter referred to as xe2x80x9cpolysiliconxe2x80x9d) is used at least for the driving transistors, though not necessarily for the pixel transistors. When a polysilicon film is to be formed integrally over the display area and the driving circuit area of the TFT panel, the typical method is as follows. An amorphous silicon film is formed on the substrate by reduced pressure CVD or the like, and the amorphous silicon is annealed by excimer laser beam irradiation, so that the amorphous silicon is crystallized to be a polysilicon film. It is necessary for the excimer laser beam to maintain uniform energy density, and to be incident on as wide as possible a range. Therefore, a beam having a linear cross section, with a length of 150 to 300 mm and a width of 300 xcexcm, that is, almost a line of 0.3 mm, has been dominantly used. In case a plane is to be irradiated, the longitudinal direction of the beam is set in the Y-direction of the coordinate system shown in FIG. 28, the thin width direction of the cross section is aligned with the X-direction, and the plane is scanned by pulse-exposure with a frequency period of about 200 to 300 Hz with the beam shifted by 5 to 50 xcexcm shot by shot along the X-direction.
The reason why polysilicon is used for the thin film semiconductor is that it is necessary to increase mobility for electrical carriers in the driving transistor. The mobility of the electrical carriers in amorphous silicon is significantly lower than the mobility in polysilicon. Therefore, amorphous silicon cannot be used for the driving transistor. Conventionally, when the display area is not integral with the driving circuit area, it was the practice that amorphous silicon was used for the pixel transistors, and single crystal silicon was used for the driving transistors which were fabricated through separate steps. The mobility in polysilicon is smaller than that in the single crystal silicon. However, necessary mobility can be ensured by making the crystals large. Accordingly, it follows that the energy density of the laser beam mentioned above must be such that it can turn the amorphous silicon into polycrystalline silicon and, in addition, attain the appropriate size of the crystal grains. The energy density of the laser beam satisfying these conditions is limited to a narrow range having upper and lower limits, though not so limited as to cause a major difficulty in implementation.
Required performance, however, may not be satisfied as to uniformity between shots of the energy density of the laser beam. More specifically, a miss shot occurs on the order of one per several tens of thousands, and the energy density falls below a lower limit or an upper limit of the above described range. At a portion affected by a miss shot, the crystal grain diameter cannot be made sufficiently large in an area corresponding to the width of one pitch of scanning, for example, an area having the width of 15 xcexcm and a length of 150 to 300 mm. The phenomenon that the crystal grain diameter cannot be made sufficiently large occurs when the energy density of the laser beam is too low or too high. Therefore, when the laser beam is scanned along the direction of the channel length with the longitudinal direction of the laser beam cross section set parallel to the channel width direction of the TFT, the following problem arises. When there is an unsatisfactory shot in the TFT channel area, an area in which crystals are not sufficiently crystallized (hereinafter also referred to as defective area) is formed traversing the channel regions of the TFTs. As a result, a number of defective transistors are formed continuously along the area of the unsatisfactory shot, resulting in unsatisfactory display, and hence lower yield. FIG. 29 represents a two-dimensional arrangement of the conventional driving transistors which may possibly suffer from the problem. For a plurality of driving transistors 130 in the X-direction driving circuit portion, a gate interconnection 141, a source interconnection 143 and a drain interconnection 142 are formed continuously over the driving transistors. At portions where the interconnections and the driving transistors overlap, gate electrodes, source electrodes and drain electrodes are formed. In the source and drain electrodes, contact portions 137 are formed to be electrically connected to source and drain regions 133 and 132 of the semiconductor thin film, respectively. When viewed two-dimensionally, channel regions 131 almost overlap the gate electrode. In FIG. 29, the pulse laser is scanned along the direction of the channel length 135 shot by shot, with the longitudinal direction of the cross section of the laser beam arranged parallel to the direction of the channel width 134. At this time, when a miss shot happens to overlap the channel region, the miss shot corresponding to one pitch 140 of scanning completely traverses the channel regions 131. In FIG. 29, the hatched portions represent overlapping portions of channel regions 131 and the one pitch P of the miss shot of the laser beam. The charge carriers cannot pass through the channel regions of the transistors without passing through the hatched portions. The miss shot affects the transistor characteristic when one pitch 140 of the miss shot is formed completely traversing the channel regions. Even when one pitch 140 of scanning of the miss shot is formed completely traversing the source region or the drain region, the influence is not so great. In the arrangement of FIG. 29, a series of driving transistors connected to the gate interconnection are all damaged, and hence there is a significant influence clearly degrading the quality of display in the liquid crystal display apparatus. In such an arrangement of the driving transistors as shown in FIG. 29, driving transistors are accommodated in a compact manner in areas 115 and 116 as shown in FIGS. 30 and 31, and the frame of the display screen is not large. The gate interconnection 131 of the driving transistors (not shown) of the X-direction driving circuit portion does not extend beyond areas 115 and 116. Therefore, it is unnecessary to enlarge the width 118 of the left frame and the width 119 of the right frame. As the driving transistors in the X-direction driving circuit portion, pairs of n channel and p channel transistors (both not shown) are arranged. The paired transistors are not distinguished, if not particularly necessary.
In order to solve the problem of degraded display quality described above, a proposal has been made in which the direction of the channel width is made non-parallel to the longitudinal direction of the cross section of the laser beam (Japanese Patent Laying-Open No. 11-87720). Further, a proposal has been made in which the conditions of non-parallelism are limited, using the channel width W, channel length L, scan pitch P in the direction of the shorter axis of the cross section of the laser beam, and an angle xcex8 between the scanning direction of the laser beam and the direction of the channel width (Japanese Patent Laying-Open Nos. 11-87729 and 11-87670). More specifically, referring to FIG. 32, the driving transistors are inclined by an angle xcex8 with respect to the direction of the longitudinal axis of the cross section of the laser beam, such that the condition of Wxc2x7sin xcex8 greater than P is satisfied, where W represents the width 134 of the channel portion and P represents the scan pitch of the laser beam. When a miss shot occurs in the channel regions of driving transistors with a channel width W as wide as 700 xcexcm, there would be a portion with sufficient margin at which the channel region can pass, without being intercepted by the defective portion resulting from the miss shot, if this condition is satisfied. In other words, there would be a considerable number of charge carriers which pass only through areas of satisfactory characteristic. As a result, the transistor characteristics are not degraded by the laser beam miss shot, and hence substantial degradation of the display quality can be avoided.
In a driving circuit controlling the display signal, however, a large number of driving transistors are arranged continuously. Therefore, when the driving transistors are arranged in series along a prescribed direction, crossing a side of a rectangular screen, the series of driving transistors extend considerably outside of the edge of the display screen. As can be seen in FIGS. 33 and 34, when the driving transistors (not shown) are inclined, the widths 118 and 119 of the frames are much increased. Such an inclined arrangement goes against the trend of narrowing the frame in the peripheral portion of the liquid crystal display screen, undesirably widening the driving circuit areas. If the scan pitch P could be made extremely small, the condition Wxc2x7sin xcex8 greater than P will be satisfied even when the angle xcex8 is made smaller. If the scan pitch P is made too small, the polysilicon surface comes too coarse to be practically used. An arrangement in which driving transistors, placed obliquely to the longitudinal direction of the laser cross section, are arranged in series along the Y direction may be possible. This arrangement attains a narrower frame. In this arrangement, however, the distance between electrodes of transistors becomes narrow, making it difficult to form transistors aligned along one direction. Therefore, the arrangement in which the driving transistors placed obliquely are connected in series along the Y direction is not used for the actual manufacture of the driving circuit portion.
Therefore, an object of the present invention is to provide a liquid crystal display apparatus including a TFT panel having, at least in a driving circuit area, a polysilicon thin film generated from an amorphous silicon thin film by scan irradiation with excimer laser shots, wherein arrangement susceptible to degradation in display quality of the plurality of driving transistors formed on the polysilicon thin film in case of a miss shot of the excimer laser is avoided and in which narrower frame can be attained, as well as to provide a TFT panel therefor.
In the liquid crystal display apparatus of the present invention, a thin film semiconductor for the pixel transistors and a thin film semiconductor for the driving transistors may be formed integrally through the same process steps, or the thin film semiconductors may be formed through separate process steps and thereafter integrated with each other.
The liquid crystal display apparatus in accordance with the present invention includes a liquid crystal, a TFT panel driving the liquid crystal and an opposing substrate. The TFT panel has a display area in which a plurality of signal lines and a plurality of scanning lines are arranged intersecting with each other and a plurality of pixel transistors are arranged at the intersecting portions, and a driving circuit area including a plurality of driving transistors. A gate interconnection of the driving transistors formed in the driving circuit area is arranged along a folded line, or a zigzag line, having a first line extending linearly along a first direction, a second line extending linearly along a second direction different from the first direction, and a bent portion connecting the first and second lines. Further, the driving transistors are arranged along the first and second lines with the channel regions not overlapping the bent portion when viewed two-dimensionally.
In this structure, the channel regions of the driving transistors formed overlapping and below the gate interconnection are also arranged along the zigzag line. Therefore, the positions of the channel regions of the driving transistors can be shifted from the linear defective portion resulting from a laser miss shot. As a result, a problematic situation where the defective area is positioned traversing all the channel in the transistors is avoided, and the defective area, if any, overlaps an end portion of the channel region, the source region or the drain region, dependent on the positions of the driving transistors. Therefore, the undesirable influence of the defective portion can be dispersed, and substantial degradation of the display quality can be prevented. Further, as the transistors are arranged along the zigzag line having the lines extending in the first direction and the second direction, the driving circuit area does not extend much beyond the display screen, and hence a narrow frame can be realized. Further, it is also possible to arrange the central portions of the channel regions of the driving transistors along the zigzag line mentioned above and, in addition, to arrange the driving transistors inclined or rotated with respect to the direction of the longitudinal axis of the laser beam. In this case, each driving transistor is subjected to the shift in the direction of the laser beam scanning as well as inclination or rotation by a prescribed angle. This further decreases the possibility of generation of the driving transistors entirely along a line of which channel regions are completely traversed by the defective portion resulting from the laser miss shot. More specifically, when inclined or rotated, such an arrangement is attained in which the longitudinal direction of the thin cross section of the laser beam becomes parallel to the direction of the channel length, and hence portions at which the channel region can be passed through without obstructed by the defective portion increase. If the arrangement having the shift mentioned above is additionally implemented, the effect of dispersing the risk is attained and, in addition, the effect of the increase of the portions where the channel regions can be passed through without passing through the defective portion is attained. As a result, the driving transistors become almost free of any degradation in characteristics.
The channel regions are formed avoiding the bent portion of the gate interconnection when viewed two-dimensionally from the following reason. At the bent portion, there would be an electric field concentration, possibly affecting movement of the charge carriers in the channel regions, and possibly causing malfunction.
The driving circuit area may be formed integrally with the display area on one substrate through the same process steps, or the driving circuit area and the display area may be formed on separate substrates through different process steps, and thereafter integrated with each other. Therefore, the semiconductor thin film of the pixel transistor may be the amorphous silicon, or it may be the polysilicon formed integrally through the same process steps.
In the liquid crystal display apparatus of the present invention described above, the first line and the second line are arranged such that the direction from an end point closer to the display area to an end point far from the display area are reversed, when viewed from the side of the display area.
By this structure, it becomes possible to prevent the defective area from completely traversing the channel regions of all of the series of transistors, and to actually realize narrower frame. The direction or length viewed from the side of the display area is the direction of projection or the length of projection of the display area to the peripheral edge.
In the liquid crystal display apparatus of the present invention, the first and the second lines are each consist of smaller folded lines, that is, zigzag lines.
By this structure, it becomes possible to arrange the whole series of driving transistors such that the channel length direction of each driving transistor is set parallel to the direction in which a boundary between the display area and the driving circuit area extends, along the first line, for example. More specifically, it becomes possible to set the direction of interconnections including the gate interconnection parallel to the width direction of the channel region, and additionally to set the arrangement of each driving transistor freely.
In the liquid crystal display apparatus in accordance with the present invention, the bent portion includes a line connecting the first and second lines and crossing the boundary between the display area and the driving circuit area almost orthogonally.
By this structure, it becomes possible to arrange the driving transistor with sufficient margin, along the folded zigzag line.
In the liquid crystal display apparatus of the present invention, the bent portion includes a portion where the first and second lines are connected directly, with a certain angle therebetween.
This structure enables narrowing of the frame in the X-direction (the direction in which the boundary between the display area and the driving circuit area extends), as well as in the Y-direction (vertical direction of the boundary). In other words, the driving transistors are arranged with high density along the Y direction. It is preferred that the driving transistors arranged along the first and second lines are shifted in position in the X direction so as to disperse risks, that is, the transistors should not be positioned commonly aligned with respect to the defective area that is parallel to the Y direction. Further, it is preferred that the axial direction of the channel regions is much inclined from the Y direction, so as to prevent the channel directions from being completely traversed by the defective area.
In the liquid crystal display apparatus of the present invention, the width direction of the channel region of the driving transistor is arranged parallel to the first and second lines. By this structure, not only the width direction of the channel region but also the width directions of the source and drain regions are also made parallel to the direction of the source and drain interconnections, respectively, whereby formation of the interconnections is facilitated.
In the liquid crystal display apparatus of the present invention, the display area is rectangular, and the driving circuit area is arranged not to extend beyond extended lines from two parallel opposing sides of the rectangular display area.
Because of this structure, it becomes possible to realize dispersion of risks as well as narrowing of the frame, while crystallization and crystal growth are ensured without any unevenness, over the entire display screen with high efficiency, with the laser beam not unnecessarily overlap at any portion at the time of laser annealing.
In the liquid crystal display apparatus of the present invention, a distance from a driving transistor and a neighboring driving transistor positioned nearest to the first mentioned driving transistor viewed from the side of the display area is made longer than the interval of pitch stripes, which are the traces of laser beam scanning.
Because of this structure, even when a defective portion resulting from a laser beam miss shot traverses a channel region of one driving transistor, the defective portion passes through the source region or the drain region of the neighboring driving transistor. As a result, substantial influence of the defective portion on the display quality can be avoided, and the production yield can be improved.
In the liquid crystal display apparatus of the present invention, the channel region of each driving transistor is formed such that the distance between a corner of the channel region nearest to the display area and a corner of the channel region farthest from the display area when viewed from the side of the display area is made longer than the interval of pitch stripes, which are the traces of laser beam scanning.
In this structure, the driving transistors must be arranged inclined or rotated with respect to a vertical line from the aforementioned sides. When the driving transistors are arranged inclined or rotated with respect to the direction of the longer axis of the laser beam, portions where the charge carriers can pass not hindered by the defective portion increases by the reason described above, even in the worst case where the defective portion resulting from the laser beam miss shot goes through the channel portion.
In a liquid crystal display apparatus in which the distance between the corners of the channel region is longer than the interval between the pitch stripes, a portion where the charge carriers can pass through the channel region without hindered by the defective portion surely exists, even when the defective portion goes through the channel region. Therefore, the undesired influence of the defective portion on the display quality can be avoided.
In the liquid crystal display apparatus described above, the driving transistors and the pixel transistors may be formed on a polysilicon film formed integrally on one substrate. Alternatively, the driving transistors and the pixel transistors may be formed on thin film semiconductors formed through separate process steps on separate substrates and thereafter be integrated. In either type liquid crystal display apparatus, as long as the transistors in the driving circuit area are formed with the amorphous silicon turned to polysilicon, the features described above exhibit the function of preventing substantial degradation of display quality. In both types of liquid crystal display apparatuses, the semiconductor in the display area may be an amorphous silicon film as it is, or it may be a polysilicon film prepared by irradiating the amorphous silicon with the excimer laser beam integrally in the same process step.
The TFT panel of the present invention is for driving liquid crystal, having a display area including a plurality of signal lines and a plurality of scanning lines arranged intersecting with each other and a plurality of pixel transistors arranged at the intersecting portions, and a driving circuit area including a plurality of driving transistors. A gate interconnection of the driving transistors formed in the driving circuit area is arranged along a folded line, or a zigzag line, including a first line extending linearly along a first direction, a second line extending linearly along a second direction different from the first direction, and a bent portion connecting the first and second lines. Further, the driving transistors are arranged along the first and second lines with the channel regions not overlapping the bent portion, when viewed two-dimensionally.
The TFT panel having such as a structure is used in a liquid crystal display apparatus, whereby the defective portion is dispersed at the central portion and an end portion of the channel region as well as the source and drain regions. Therefore, significant degradation of display quality can be avoided. Further, when the driving transistors are arranged rotated by a prescribed angle with the channel region being the center, with the driving transistors shifted from each other as described above, the risk can further be dispersed.
The foregoing and other objects, features, aspects and advantages of the present invention will become more apparent from the following detailed description of the present invention when taken in conjunction with the accompanying drawings.